Surface properties play an important role in the initial growth of films in thin-film processes. The increasing need for sophisticated film preparation processes, including epitaxial growth, selective growth, trench filling, etc., requires that the surface be uniform and well defined. Ideally, surface preparation techniques should be optimized for each particular film deposition process.
In particular, especially for deposited thin-films, the surface state before deposition directly impacts the interface properties between the surface and the thin film deposited. For example, different wafer surfaces, such as tetraethylorthosilicate (TEOS), borophosphosilicate glass (BPSG), or silicon, exhibit different nucleation and average deposition rates when silicon nitride is deposited thereon; however, once a uniform layer of nitride is convering the entire surface, the instantaneous deposition rate should be independent of the original surface. Further, for example, when silicon is exposed to air, native oxide is formed on the surface of silicon which may decrease the nitride deposition rate and inhibit the proper termination of silicon bonds at the silicon surface when a silicon nitride thin film surface is deposited thereon. The affected interface properties may degrade the isolation performance or dielectric quality of silicon nitride films deposited on the various surfaces.
Silicon nitride (Si.sub.3 N.sub.4) deposition is important to the fabrication of integrated circuits because silicon nitride films act as diffusion barriers and have unique dielectric qualities. For example, high-quality dielectrics formed using silicon nitride films are used in the fabrication of MOSFET gates, memory cells, and precision capacitors. The interface between the substrate upon which the silicon nitride is deposited and the silicon nitride film, at least in part, defines the isolation and dielectric characteristics of the devices utilizing the silicon nitride film.
In a conventional silicon nitride deposition method 10 as represented in FIG. 1 upon a silicon surface (including single crystal, poly, epitaxial, etc.), the surface upon which the silicon nitride layer is to be deposited is normally pretreated such as by removing the native oxide using HF solutions and/or HCL solutions. A film of silicon nitride is then deposited on the pretreated surface such as by the reaction of silane with ammonia. Unless the pretreatment and silicon nitride deposition are performed in a cluster tool for controlling contamination, some native oxide may be present on the surface when the silicon nitride deposition is performed. The presence of native oxide degrades device performance and although the use of cluster tools reduces the native oxide growth, cluster tools reduce throughput of wafers and are generally more costly to operate as compared to standard processing equipment, such as conventional deposition reactors.
Moreover, as mentioned above, different nucleation and deposition rates occur for the deposition of silicon nitride on different wafer surfaces, such as TEOS, BPSG, or silicon. This leads to different or degraded electrical characteristics of the devices fabricated using a silicon nitride deposited layer on different wafer surfaces. In addition, when silicon nitride is deposited, an incubation time occurs at the start of the deposition process wherein there is no apparent deposition of silicon nitride on the wafer surface. Such differing nucleation and deposition rates and also the incubation period result in degraded electrical characteristics of the semiconductor devices being fabricated.
For the reasons indicated above and for other reasons which will become apparent from the detail below, improved methods of forming silicon nitride films are needed to improve the characteristics of the semiconductors devices fabricated, and also to reduce the cost and increase the throughput for fabricating such devices.